Ellingham diagrams free#

The forward reaction for aluminum oxide is 4/3Al + O 2 = 2/3Al 2O 3 with a О”G o of -900 KJ. At 700В° C, this reaction has a О”G o of +580 KJ. Consider the reverse reaction for chromium oxide, 2/3Cr 2O 3 = 4/3Cr + O 2. Now, since the О”G o-T line for the Al 2O 3 reaction lies below the О”G o-T line for Cr 2O 3 reaction, it is possible to reduce chromium oxide to metallic Cr by Al metal.

Ellingham diagrams free#

So instead of having to calculate the standard free energy of formation for a particular oxidation reaction at some temperature of interest, it is easy to find it on the Ellingham diagram. Also, at 700В° C the О”G o for the Cr 2O 3 reaction is -580 KJ. At a temperature of 700В° C, it can be seen from the dotted lines on the diagram that the standard free energy of formation (О”G o) for the aluminum oxide reaction is -900 KJ. For example, consider the oxidation reaction for Al (see Fig. But there is valuable quantitative information that can be extracted from the diagram as well. So far, we have only been looking at the qualitative attributes of the Ellingham diagram. By contrast, the higher an oxidation reaction line is on the diagram, the easier it is to reduce the oxide.Д®xtracting Quantitative Information from the Ellingham Diagram One can also say the Ca has a higher affinity for oxygen compared to Ti, Si and Cr. So one can say that at any given temperature, calcium oxide is more stable than chromia, silica or titania. The TiO 2 reaction line is lower than the SiO 2 line, which in turn is lower than the Cr 2O 3 line. The more negative the free energy of formation (О”G o) for a given reaction, the lower the line representing the reaction will be on the diagram. The fundamental beauty of the Ellingham construction is that one can directly visualize the relative affinity of the metals for oxygen and relative stability of the oxides by simply looking at the positions of the lines on the diagram. Each of the О”G o-T lines represent the relative stability of the oxide formed. Since most of these reactions are for a solid + a mole of oxygen producing a solid oxide, the entropy change for the reactions is similar, so the slopes of the lines are relatively parallel. We also know that the slopes of the О”G o-T lines are given by вЂ“ О”S o (the negative of the standard entropy change). We know that these О”G o as a function of T lines are linear over the range where no phase changes occur, that is О”G o is approximately linear with temperature. As with the original Ellingham diagram, the oxide reactions represented here all use one mole of oxygen gas. I like this version because it is very clean and includes the nomographic scales for oxygen partial pressure, along with the pCO/ pCO 2 and pH 2/ pH 2O ratios. David Robert Gaskell, who was a Professor of Metallurgical Engineering at Purdue University. Practical Applications of the Ellingham DiagramДЇigure 1 is a modern version of the Ellingham diagram, which was graciously provided by the late, great Dr. These are all important principles to understand heat treating steel and aluminum. Now we move on to a more detailed look at how the Ellingham diagram can be utilized in practical situations. Previously, I introduced the fundamental precepts that define the construction of the Ellingham diagram, which is based on the Gibbs free energy at standard state conditions (О”G o) for a metal oxidation reaction.